This invention relates to a charged particle beam column for the examination of specimen. In particular, this invention relates to a beam column where the beam may land on the specimen surface under an oblique landing angle.
In charged particle beam devices, such as a scanning electron microscope (SEM) the typical aperture angle as well as the typical landing angle of the charged particle beam is of the order of several millirads. For many applications it is desirable that the charged particle beam lands on the sample surface under a much larger angle of typically 5xc2x0 to 10xc2x0, corresponding to 90 to 180 millirads. Some uses require tilt angles in excess of 15xc2x0 or even 20xc2x0.
One application which requires large landing angles is the stereoscopic visualization of a specimen surface. Stereographic techniques using a SEM date back to the early developmental period of scanning electron microscopy. Since electrons can be collected from practically all parts of a relatively rough sample, a SEM image has a rather xe2x80x9crealxe2x80x9d appearance. The main reason for this real appearance is that the secondary electron signal produced at the point of beam impact varies with the local slope of the surface in the same way as the perceived brightness of the surface of a diffusely illuminated macroscopic object. Furthermore, variations in the efficiency with which this signal is collected by the weak electric field from the detector modifies the signal as a function of position such that it appears as if the sample surface contained shadows. While the images have thus all the visual cues of a conventional black and white photograph, these cues are in many situations deceptive. It is therefore essential that a method which provides authentic perspective information is available. Stereoscopic visualization is such a method. It is useful and sometimes indispensable for detecting and resolving situations where other coding mechanisms yield ambiguous results.
In another application, topographical information about the specimen surface may be extracted, for example, from the parallax between stereo pairs of images obtained with a tilted beam. A further application, three-dimensional imaging of a specimen, requires also a beam tilted by several degrees, see, e.g., U.S. Pat. No. 5,734,164.
In all these applications, the beam tilting mechanism plays a key role. In early solutions, a stereo effect was achieved by mechanically tilting the specimen to provide two perspectives. However, due to mechanical imperfections, a lateral movement of the specimen is inevitable, which often results in misregistrations between the elements of a stereo image pair. This problem is especially pertinent for highly regular structures such as an array of memory cells in an integrated circuit.
When beam tilting is carried out electrically, the fact that the specimen can remain horizontally is a significant advantage as far as the lateral coordinate registration is concerned. Electrical tilting is also much faster than its mechanical counterpart. The electrical method, however, has also certain drawbacks. In one method, the beam is deflected above the objective lens (pre-lens deflection) in such a way that each ray seems to emerge from a point coincident with the apparent position of the electron source (see FIG. 3). This way, each ray is focussed on the same area of the sample as long as the sample surface is in focus. However, as a consequence, the beam traverses the field of the objective lens considerably off-axis with its attendant degradations due to lens aberrations. Especially chromatic aberrations limit the attainable resolution to several tens of nanometers. Many applications require a much higher resolution of about 5 nm.
If, as in another method, the deflection coils are arranged below the objective lens (post-lens deflection), the beam passes through the lens on the optical axis (FIG. 3). However, the physical dimensions of the coils below the final lens imposes a limit on the minimum attainable working distance, i.e., on the minimum attainable distance between the final lens and the specimen to be examined. An acceptable resolution is then not achieved due to the degraded instrument resolution arising from the enlarged working distance.
The present invention intends to overcome the above-mentioned drawbacks and disadvantages of the prior art. Specifically, the invention intends to provide an improved charged particle beam column allowing specimen to be examined with an obique beam landing angle while maintaining a high resolution of the charged particle image. According to one aspect of the present invention, to achieve this, there is provided a column as specified in claim 1 and a method as specified in claim 13.
Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach to define the invention in general terms.
According to one aspect, the invention provides a column for directing a beam of charged particles with a finite energy spread onto a specimen surface under an oblique beam landing angle, the column comprising: a particle source for providing the beam of charged particles propagating along an optical axis; an objective lens for focussing the beam of charged particles onto the specimen surface; a deflection unit for deflecting the beam of charged particles away from the optical axis such that the beam of charged particles traverses the objective lens off-axis, thereby causing a chromatic aberration, a compensation unit adapted to disperse the beam of charged particles, thereby substantially compensating said chromatic aberration in the plane of the specimen surface, whereby the combined action of the objective lens and the deflection unit directs the beam of charged particles to hit the specimen surface under said oblique beam landing angle.
As discussed hereinbefore, the deflection leads to an off-axis path of the beam through the objective lens which gives rise to large chromatic aberrations due to the finite energy spread of the beam. It has surprisingly been found by the present inventors that this first chromatic aberration caused by the deflection can be compensated in the plane of the specimen surface by adding an element which introduces a second chromatic aberration of substantially the same kind and magnitude as the first chromatic aberration but which is substantially in the opposite direction. Such a second chromatic aberration may be introduced by dispersing the beam of charged particles.
In a preferred embodiment the compensating element comprises means for generating crossed electrostatic and magnetic deflection fields. Preferably, the crossed electrostatic and magnetic fields are created substantially perpendicular to the optical axis and form a so-called Wien filter. The compensation unit is advantageously in the form of an electrostatic and magnetic multipole (2n-pole, with n=1, 2, 3 . . . ), preferably selected from the group consisting of electrostatic and magnetic dipole (2-pole), quadrupole (4-pole), hexapole (6-pole) and octupole (8-pole).
In a further preferred embodiment, the electrostatic and magnetic 2n-pole comprises 2n pole pieces and 2n electrodes which are distinct from said pole pieces. The pole pieces and the electrodes are arranged in a plane perpendicular to the optical axis. In a still further preferred embodiments, the electrostatic and magnetic 2n-pole comprises 2n pole pieces, wherein each of the 2n pole pieces is adapted to be used at the same time as an electrode. The pole pieces are arranged in a plane perpendicular to the optical axis.
Without being bound to a particular theory, the compensating effect of a Wien filter in the column is presently understood as follows:
For a certain beam landing angle, for example 5xc2x0, the necessary deflection causes the center of the beam to pass the objective lens at a certain distance from the optical axis. Then, the focal length of the objective lens depends on the energy of the charged particles and on the distance between the trajectory of the beam center and the optical axis. Since the beam of charged particles has a finite energy spread, particles with different energies are deflected by the lens in slightly different direction, causing the chromatic aberration of the lens (see FIG. 3).
In the Wien filter, the electric field E and the magnetic field B generate an electric and a magnetic force on the charged particles, Fel=qE, and Fmag=q (vxc3x97B), wherein q=xe2x88x92e is the electron charge. If the electric and magnetic field are perpendicular to each other and to the velocity of the charged particle, the electric and magnetic forces are in opposite directions. For particles with a certain velocity, v=|E|/|B|, the net force is zero, and they pass the filter unaffected. Particles with a different speed experience a net force F=|Felxe2x88x92Fmag| and are deflected by the Wien filter. In effect, a beam of charged particles with a finite energy spread passing the Wien filter is dispersed, as particles with different energies are deflected by different amounts.
The dispersion leads to an at least partial compensation of the chromatic aberration of the objective lens. The invention has thus the advantage that large beam landing angles on the sample surface can be provided without the usual reduction in resolutions arising from large chromatic aberrations.
In the case where the compensation unit is formed by an electrostatic and magnetic 2n-pole with n being at least 2, both magnetic and electrostatic fields can be adjusted to deflect in an arbitrary direction in the plane perpendicular to the optical axis. Thereby, a compensation can be achieved for any direction of the deflecting action.
In the case where the compensation unit is formed by an electrostatic and magnetic 2n-pole with n being at least 3, more homogeneous deflection fields may be generated. This is especially important, if the fields are to be strong, if the beam diameter in the Wien filter is large, or if the charged particle beam is allowed to pass the filter off-axis. Additionally, higher order deflection fields can be generated which reduce or compensate the coma of the objective lens, which forms the second largest tilt aberration.
If the pole pieces are used at the same time as electrodes, electric and magnetic field with substantially identical spatial distribution are generated. The excellent matching of the fields is important when the deflection fields are to be very strong.
In a preferred embodiment, the deflection unit is adapted to provide a beam landing angle less than 25xc2x0, preferably between 3xc2x0 and 15xc2x0, more preferable between 5xc2x0 and 10xc2x0.
In a further preferred embodiment, the deflection unit comprises two deflectors adapted to deflect the beam of charged particle away from the optical axis to a path seeming to emerge from a point coincident with the apparent position of the particle source or, if applicable, to emerge from a point coincident with the apparent position of an intermediate image of the particle source.
In a still further preferred embodiment, the compensation unit is arranged between the particle source and the deflection unit. In certain cases it may be advantageous, to arrange the compensation unit within the deflection unit. Even though these two arrangement are preferred, it is also possible to arrange the compensation unit below the deflection unit.
Although the deflection system described so far can be used with any kind of objective lens, in a further aspect of the invention, the objective lens is a compound magnetic-electrostatic lens. Preferably, the electrostatic part of the compound magnetic-electrostatic lens is an electrostatic retarding lens. Using such a compound magnetic-electrostatic lens yields superior resolution at low acceleration energies, such as a few hundred electron volts in case of a SEM. Such low acceleration energies are desirable especially in modern semiconductor industry, to avoid charging and/or damaging of radiation sensitive specimens. In a preferred embodiment, the electrostatic retarding lens reduces the energy of a beam of electrons as charged particles to less then 5 keV, more preferably to less then 2 keV, most preferably to about or less than 1 keV.
According to an especially preferred aspect of the invention, the objective lens is a magnetic immersion lens.
In a preferred embodiment, the column comprises means for applying a potential difference between the specimen and a pole piece of the objective lens. An electrostatic retarding lens may thus be created between the specimen and a pole piece of the objective lens, without making additional electrodes necessary. The skilled person will appreciate, however, that additional electrodes may be present to supplement and/or modify the thus generated retarding field.
Preferably, the column further comprises means for scanning the beam of charged particles over the surface of the specimen.
The invention further comprises a method for directing a beam of charged particles with a finite energy spread onto a specimen surface under an oblique beam landing angle, the method comprising the steps of:
a) providing a beam of charged particles with a finite energy spread propagating along an optical axis;
b) focussing the beam of charges particles onto the specimen surface with an objective lens; whereby the method is characterized in further comprising the steps of
c) selecting a beam landing angle;
d) deflecting the beam of charged particles propagating along the optical axis away from the optical axis such that the beam of charged particles traverses the objective lens off-axis, thereby causing a first chromatic aberration, whereby the magnitude of the deflection is chosen such that the combined action of deflecting and focussing the beam directs the beam of charged particles to hit the specimen under said large beam landing angle;
e) dispersing the beam of charged particles, thereby introducing a second chromatic aberration of substantially the same kind and magnitude but in substantially opposite direction as said first chromatic aberration to substantially compensate said first chromatic aberration in the plane of the specimen surface.
Preferably step e) of the method comprises: generating within a region along the optical axis crossed electrostatic and magnetic fields substantially perpendicular to the optical axis and to each other; and passing the beam of charged particles through said region.